1. Field of the Invention
The present invention relates to a method for modifying the surface of a counter electrode, and a counter electrode whose surface is modified by the method. More specifically, the present invention relates to a method for modifying the surface of a counter electrode by treating the surface of the counter electrode with a polyethylene glycol derivative having a pendant group at one end, thereby achieving an increased electron transfer rate at the interface between the counter electrode and an electrolyte layer of a photovoltaic device and improved affinity of the counter electrode for the electrolyte layer; and a counter electrode whose surface is modified by the method.
2. Description of the Related Art
Since solar cells, which constitute one type of photovoltaic device for converting solar energy into electric energy, utilize virtually inexhaustible solar energy, unlike other energy sources, and are environmentally friendly, they are gradually gaining importance. Particularly, when solar cells are used as power sources in portable digital communication devices, such as portable computers, cellular phones, and personal digital assistants (PDAs), they are expected to be charged by solar power only.
Monocrystalline or polycrystalline silicon-based solar cells have extensively been used. However, silicon-based solar cells require the use of large, expensive equipment and costly raw materials, incurring considerable fabrication costs. In addition, silicon-based solar cells suffer from numerous difficulties in improving the conversion efficiency of solar energy into electric energy.
Under such circumstances, there has been increasing interest in solar cells using organic materials that can be fabricated at reduced costs. Particularly, dye-sensitized solar cells have received a great deal of attention owing to their low fabrication costs.
Dye-sensitized photovoltaic devices are photoelectrochemical solar cells which comprise, for example, a semiconductor electrode including a transparent electrode, a porous semiconductor layer formed of nanoparticles adhered to the transparent electrode, and a dye coated on the surface of the porous semiconductor layer; a counter electrode arranged opposite to the semiconductor electrode; and a redox electrolytic solution disposed in a space between the two electrodes. The advantages of dye-sensitized solar cells are high power conversion efficiency and low fabrication costs.
However, since dye-sensitized solar cells are wet cells using a liquid electrolyte, leakage or volatilization of the electrolyte solution may occur during long-term use of the solar cells, causing problems of low reliability and poor long-term stability (e.g., rapid decrease in power conversion efficiency) of the solar cells.
To solve these problems associated with wet solar cells, research has been conducted to replace liquid electrolytes with solid electrolytes or quasi-solid hole conductors. One of the first solar cells using a solid polymer electrolyte as a hole transport material was developed by De Paoli's research group in Brazil in 2001, and thereafter, there has been much research aimed at the development of solar cells using solid polymer electrolytes.
However, solar cells using polymer electrolytes have low energy conversion efficiencies, which makes them undesirable for commercialization. Further, solar cells using polymer electrolytes have low ionic conductivities as compared to solar cells using liquid electrolytes. When the polymer electrolyte of the solar cell includes a polymer having a long molecular chain, it is difficult for the polymer electrolyte to infiltrate into pores formed between the nanometer-sized semiconductor particles. In addition, if the polymer electrolyte insufficiently surrounds the semiconductor nanoparticles and does not provide a complete connection to the semiconductor nanoparticles without any electrical shorting, the current density of the solar cell is greatly decreased. Alternatively, when the polymer electrolyte of the solar cell includes a liquid- or wax-phase polymer having a short molecular chain, the mechanical properties of the solar cell can be deteriorated and leakage of the electrolyte inevitably occurs as in wet solar cells.
A dye-sensitized solar cell includes two interfaces (i.e., a semiconductor electrode/electrolyte interface and a counter electrode/electrolyte interface). The performance of the solar cell is largely dependent upon the electron transfer rate and reduction rate at the interfaces. Particularly, since a solid polymer and a metal constitute the counter electrode/electrolyte interface of the solar cell, incomplete contact between the solid materials is caused. This incomplete contact makes the transfer of electrons between the counter electrode and the electrolyte layer difficult as compared to when a liquid electrolyte is used, thus leading to a reduction in the power conversion efficiency of the solar cell.